1. Field of the Invention
The present invention relates to a system for directing an apparatus, such as a crane bridge, which moves on wheels along rails.
2. Description of the Related Art
In a situation where apparatuses, such as crane bridges which are elongated in the transverse direction with respect to rails, are driven along the rails at a distance from one another, the wheels of the apparatus to be driven are maintained in the middle of the rails by means of mechanical guide rollers. The guide rollers provide some freedom of action so that the mechanical elasticity and deflections of the apparatus to be driven will be managed. However, for cranes, in particular, where the span length of the bridge is long and the driving speed is high, the wear of mechanical guide rollers or other such structural parts is a significant problem.
When bridge driving is accomplished with two or more motor drives with a precise speed control, it is often necessary to compensate the speeds specifically for each motor drive because there are generally many differences in the speed directions and the actual speed of the drives such that the crane bridge tends to be driven aslant. The speed differences between the ends of the bridge are due to both mechanical factors (e.g. differences in wheel dimensions because of wear, for example) and electrical factors (small differences in speed directions because of component tolerances and signal routes, for example).
These problems are attempted to be solved by the control systems for even driving a crane bridge disclosed in references GB-A-2 112 548, DE-A-25 28 293 and DE-A-28 35 688, for example, where the distance of the end of the bridge from the rail is measured with two separate detectors. The controller directs the difference of the distance measurement of the detectors to a desired value in such a manner that the crane bridge will move straight.
However, the controllers of the prior art control systems will still typically be driven on guide rollers, either inside or outside the bridge and are not able to drive the ends of the crane bridge along a desired truck.